Directional drilling is well known in the art and commonly practiced. Directional drilling is generally practiced using a bottom-hole assembly connected to a drill string that is rotated at the surface using a rotary table or a top drive unit, each of which is well known in the art. The bottom-hole assembly generally includes a positive displacement drill motor that drives a drill bit via a “bent” housing that has at least one axial offset of around 4 degrees. A measurement-while-drilling (MWD) tool connected to a top of the drill motor provides “tool face” information to tracking equipment on the surface to dynamically determine an orientation of a subterranean bore being drilled. The drill string is rigidly connected to the bottom-hole assembly, and rotation of the drill string rotates the bottom-hole assembly.
To drill a linear bore segment, the drill string is rotated at a predetermined speed while drilling mud is pumped down the drill string and through the drill motor to rotate the drill bit. The drill bit is therefore rotated simultaneously by the drill motor and the drill string to drill a substantially linear bore segment. When a nonlinear bore segment is desired, the rotation of the drill string is stopped and controlled rotation of the rotary table or the top drive unit and/or controlled use of reactive torque generated by downward pressure referred to as “weight on bit” is used to orient the tool face in a desired direction. Drill mud is then pumped through the drill string to drive the drill bit, while the weight of the drill string supported by the drill rig is reduced to slide the drill string forward into the bore as the bore progresses. The drill string is not rotated while directional drilling is in progress.
However, this method of directional drilling has certain disadvantages. For example: during directional drilling the sliding drill string has a tendency to “stick-slip”, especially in bores that include more than one nonlinear bore segment or in bores with a long horizontal bore segment; when the drill string sticks the drill bit may not engage the drill face with enough force to advance the bore, and when the friction is overcome and the drill string slips the drill bit may be forced against the bottom of the bore with enough force to damage the bit, stall the drill motor, or drastically change the tool face, each of which is quite undesirable; and, rotation of the drill string helps to propel drill cuttings out of the bore, so when the drill string rotation is stopped drill cuttings can accumulate and create an obstruction to the return flow of drill mud, which is essential for the drilling operation. Furthermore, during directional drilling the reactive torque causes the stationary drill string to “wind up”, which can also drastically change the tool face.
Therefore there exists a need for a method and apparatus for directional drilling that permits the drill string to be rotated without sacrificing directional control of the drill tool face.